Positive electrode material, electrochemical device and preparation method of positive electrode material

Abstract

The invention provides a positive electrode material, an electrochemical device and a preparation method of the positive electrode material, and belongs to the technical field of secondary batteries, the positive electrode material comprises a matrix and a coating layer, the matrix comprises LiMnxFeyMzPO4, x is more than 0.5 and less than or equal to 1.0, y is more than 0 and less than or equal to 0.5, z is more than or equal to 0 and less than or equal to 0.2, and x + y + z is more than or equal to 0.8 and less than or equal to 1.2, and an M element comprises at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al and Ca; the coating layer comprises amorphous carbon; in the particles of the positive electrode material, the ratio # imgabs0 # of the Mn / Fe molar ratio at a depth D1 from the surface of the particles to the Mn / Fe molar ratio at a depth D2 from the surface of the particles is 0.9 to 1.1; the depth D1 is R / 10, the depth D2 is 17R / 20, and R is the radius of the particle. In the lithium manganese iron phosphate matrix of the positive electrode material provided by the invention, Mn and Fe atoms are uniformly distributed, so that the crystal structure of the lithium manganese iron phosphate material can be stabilized, Mn ions are not easy to distort, and the discharge capacity, the rate capability and the cycle performance of the positive electrode material are effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a cathode material, an electrochemical device, and a preparation method of the cathode material. Background Art

[0002] As an efficient energy storage device, lithium-ion batteries have been widely used in the fields of portable electronic digital products and electric vehicles, and play an increasingly important role in today's society.

[0003] As a cathode material for batteries, lithium iron manganese phosphate has significant advantages of high energy density, stability, safety, high cost performance, and environmental friendliness, and has received extensive attention in recent years. However, in the lithium iron manganese phosphate materials prepared by the existing sintering process, the distribution of Mn and Fe atoms is uneven, which will deteriorate the distortion phenomenon of the MnO6 octahedron in the material itself, resulting in difficulties in Li ion deintercalation and Mn ion dissolution phenomena. This leads to problems such as voltage plateau and decline in cycle performance during the use of the battery.

[0004] Therefore, it is necessary to design a cathode material, an electrochemical device, and a preparation method of the cathode material to solve the above problems. Summary of the Invention

[0005] In view of the above disadvantages of the prior art, the present invention provides a cathode material, an electrochemical device, and a preparation method of the cathode material, which are used to solve the technical problems of the decline in the discharge capacity, rate performance, and cycle performance of the lithium iron manganese phosphate materials prepared by the existing process due to the uneven distribution of Mn and Fe atoms.

[0006] To achieve the above object and other related objects, the present invention provides a cathode material, which includes a matrix and a coating layer.

[0007] Among them, the matrix includes LiMn x Fe y M z PO4, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.2, 0.8 ≤ x + y + z ≤ 1.2, and the M element includes at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca; the coating layer includes amorphous carbon;

[0008] Moreover, in the particles of the cathode material, the ratio of the Mn / Fe molar ratio at a depth D1 from the particle surface to the Mn / Fe molar ratio at a depth D2 from the particle surface is 0.9 to 1.1; the depth D1 is R / 10, the depth D2 is 17R / 20, and the R is the radius of the particle.

[0009] In an example of the present invention, the substrate satisfies 0 < z ≤ 0.2; and in the particles of the positive electrode material, the ratio of the molar ratio of M / (Fe + Mn) at a depth D1 from the particle surface to the molar ratio of M / (Fe + Mn) at a depth D2 from the particle surface is 0.9 to 1.1.

[0010] In an example of the present invention, the mass content of the coating layer in the positive electrode material is 1.0 - 3.0 wt%.

[0011] In an example of the present invention, the particle size of the positive electrode material is 100 - 600 nm.

[0012] In an example of the present invention, the positive electrode material is doped with Ti, and the doping content of Ti in the positive electrode material is 1500 - 6000 ppm.

[0013] The present invention also provides a preparation method of the positive electrode material described in any one of the above examples. The preparation method of the positive electrode material includes:

[0014] Using the coprecipitation method, preparing a manganese iron phosphate precursor doped with M element or not doped with M element according to a set component ratio;

[0015] Mixing and sintering the manganese iron phosphate precursor with a lithium source and a carbon source to obtain a positive electrode material;

[0016] Among them, the M element includes at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca.

[0017] In an example of the present invention, the step of using the coprecipitation method to prepare a manganese iron phosphate precursor not doped with M element according to a set component ratio includes: mixing a manganese source, an iron source, and a phosphorus source in a molar ratio of Mn:Fe:P of (10 - 20):(0.01 - 10):20, and performing a coprecipitation reaction to obtain a manganese iron phosphate precursor not doped with M element.

[0018] In an example of the present invention, the step of using the coprecipitation method to prepare a manganese iron phosphate precursor doped with M element according to a set component ratio includes: mixing a manganese source, an iron source, an M source, and a phosphorus source in a molar ratio of Mn:Fe:M:P of (10 - 20):(0.01 - 10):(0.01 - 4):20, and performing a coprecipitation reaction to obtain a manganese iron phosphate precursor doped with M element.

[0019] In an example of the present invention, the manganese iron phosphate precursor is NH4Mn x Fe y M z PO4 or Mn x Fe yM z PO4·H2O, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.2, and 0.8 ≤ x + y + z ≤ 1.2.

[0020] In an example of the present invention, the mixing and sintering of the manganese iron phosphate precursor with a lithium source and a carbon source to obtain a positive electrode material includes: mixing the manganese iron phosphate precursor, the lithium source, and the carbon source for primary sintering to obtain a composite material of a matrix and carbon, where the matrix is lithium manganese iron phosphate doped or undoped with element M; performing secondary sintering on the composite material to form a coating layer on the surface of the matrix, such that the coating layer covers at least a part of the surface of the matrix, thereby obtaining a positive electrode material, and the coating layer is a carbon coating layer.

[0021] The present invention also provides an electrochemical device, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, and the positive electrode active material layer includes the positive electrode material described in any one of the above examples, or the positive electrode material prepared by the preparation method described in any one of the above examples.

[0022] The present invention provides a positive electrode material. In the lithium manganese iron phosphate matrix of this positive electrode material, Mn and Fe atoms are evenly distributed, which can stabilize the crystal structure of the lithium manganese iron phosphate material, making it difficult for Mn ions to undergo distortion and avoiding hindrance to the insertion and extraction of Li ions in the material, thereby effectively improving the discharge capacity, rate performance, and cycle performance of the positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other embodiments can be obtained based on these drawings without creative efforts.

[0024] Figure 1 It is a schematic flow chart of the preparation method of the positive electrode material in an embodiment of the present invention;

[0025] Figure 2 It is a schematic flow chart of step S2 in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand the other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other. It should also be understood that the terms used in the embodiments of the present invention are for the purpose of describing specific embodiments, rather than limiting the protection scope of the present invention. The test methods without specific conditions noted in the following embodiments are generally carried out under conventional conditions or according to the conditions recommended by each manufacturer.

[0027] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" cited in this specification are only for the convenience of clear description and are not used to limit the scope of implementation of the present invention. The change or adjustment of their relative relationship, without substantial change in the technical content, should also be regarded as the scope of implementation of the present invention.

[0028] The positive electrode material is one of the key materials that determine the performance of secondary batteries, directly affecting the energy density, cycle life, rate performance and safety performance of the battery. Olivine-type lithium iron manganese phosphate materials have the advantages of high energy density, low cost, environmental friendliness, safety and stability, and have received extensive attention in recent years. However, during the charge and discharge process of lithium iron manganese phosphate materials, Mn 2+ will transform into Mn 3+ , and Mn 3+ is likely to cause the shape distortion of the MnO6 octahedron in the material, resulting in defects and vacancies in the material, hindering the insertion and extraction of Li + in the material, reducing the ion migration rate, and affecting the electrical conductivity and rate performance of the material; at the same time, it will also cause the dissolution of Mn ions and deposition on the surface of the negative electrode, damaging the SEI film (Solid Electrolyte Interface) on the surface of the negative electrode, continuously regenerating and repairing the SEI film, and then consuming a large amount of active lithium, resulting in battery capacity loss.

[0029] The inventors have found through research that if the distribution of Fe and Mn atoms in the lithium iron manganese phosphate material is uneven, it will further deteriorate the distortion of the MnO6 octahedron and the dissolution phenomenon of Mn ions in the material, which will cause a further decline in the discharge capacity, rate performance and cycle performance of the battery, and deteriorate the electrochemical performance of the battery.

[0030] To solve the above problems, the present application provides a cathode material. In the lithium iron manganese phosphate matrix of the cathode material, the distribution uniformity of Mn and Fe atoms is good, which can stabilize the crystal structure of the lithium iron manganese phosphate material, making it difficult for Mn ions to distort, and avoiding the hindrance of Li ion insertion and extraction in the material, thereby effectively improving the discharge capacity, rate performance, and cycling performance of the cathode material.

[0031] The above-mentioned cathode material includes a matrix and a coating layer. The matrix is a lithium iron manganese phosphate material doped with M element or not doped with M element, and the matrix includes LiMn x Fe y M z PO4, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.2, 0.8 ≤ x + y + z ≤ 1.2, and the M element includes at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca; the coating layer coats at least part of the surface of the matrix particles, and the coating layer includes an amorphous carbon material, and the amorphous carbon can protect the surface structure of the matrix particles and improve the conductivity of the cathode material.

[0032] In the above-mentioned cathode material, the distribution uniformity of Mn and Fe elements in the cathode material particles is good. This is specifically manifested as the molar ratio of Mn and Fe elements tending to be consistent at different depths in the cathode material particles. For example, the ratio of the Mn / Fe molar ratio at a depth D1 from the particle surface to the Mn / Fe molar ratio at a depth D2 from the particle surface is 0.9 to 1.1, and optionally, it can be 0.98 to 1.05; where the depth D1 is R / 10, the depth D2 is 17R / 20, and R is the radius of the cathode material particles. The above parameters indicate that in the cathode material particles, the Mn / Fe molar ratio near the particle surface is relatively close to the Mn / Fe molar ratio near the particle center. It can be seen that the ratios of Mn and Fe elements at different depths in the cathode material particles tend to be consistent, and the distribution of Mn and Fe atoms in the cathode material particles is relatively uniform.

[0033] In the cathode material particles, the good distribution uniformity of Mn and Fe atoms can effectively stabilize the crystal structure, avoid the cell distortion caused by the excessive content of Mn atoms in local areas of the material particles, and thus effectively reduce the defects and vacancies in the cathode material, improve the insertion and extraction efficiency of Li ions in the material, and increase the rate performance of the battery; at the same time, it can also inhibit the loss of active substances caused by the dissolution of Mn ions during charge and discharge, and improve the discharge capacity and cycling performance of the battery.

[0034] In some embodiments, the lithium iron manganese phosphate material of the matrix is doped with M element, and the matrix includes LiMn x Fe y Mz PO4, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 < z ≤ 0.2, and 0.8 ≤ x + y + z ≤ 1.2. Among them, the M element is selected from at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca. That is, the M element can be any one of the above-listed element types, such as Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, or Ca, etc.; the M element can also be any combination of two or more of the above-listed element types. For example, the M element can be a combination of Ni and Ti, or a combination of Ni and Mg, or a combination of Ti and Mg, or a combination of Cu and Zn, or a combination of Al and Zn, or a combination of Ca and Mo, or a combination of Co, Ni, and Cu, or a combination of Ni, Ti, and Mg, or a combination of V, Cr, or Sc, or a combination of Ni, Al, and Ga, or a combination of Ni, Ti, W, and Mg, etc., and will not be listed one by one here. In addition, when the M element is a combination of two or more, the ratio of each element within the combination is not restricted. In other embodiments, the M element can also be an element type not listed above.

[0035] In addition, in the above embodiments, the M element doped in the matrix is also uniformly distributed in the cathode material particles. The uniformly doped M element can effectively enhance the structural stability of the cathode material and improve the electrochemical performance of the cathode material. In the cathode material particles, the molar ratios of each element at different depths tend to be consistent. For example, the ratio of the M / (Fe + Mn) molar ratio at a depth D1 from the particle surface to the M / (Fe + Mn) molar ratio at a depth D2 from the particle surface is 0.9 to 1.1, and optionally, it can be 0.98 to 1.05; where the depth D1 is R / 10, the depth D2 is 17R / 20, and R is the radius of the cathode material particle. The above parameters indicate that in the cathode material particles, the M / (Fe + Mn) molar ratio near the particle surface is relatively close to the M / (Fe + Mn) molar ratio near the particle center. It can be seen that the element ratios of M to Mn and Fe at different depths in the cathode material particles tend to be consistent, and the distribution of M atoms in the cathode material particles is relatively uniform.

[0036] It should be noted that in this application, the Mn / Fe molar ratio and M / (Fe + Mn) molar ratio at depths D1 and D2 from the surface of the positive electrode material particles are obtained through etching-XPS testing. The specific testing method includes grading and screening the positive electrode material to obtain test samples with different particle sizes; then, for each test sample with a screened particle size, on the premise of knowing the particle size of the test sample, the measurement sample is etched successively to depth D1 and depth D2, and XPS test spectrum analysis is performed when the test sample is etched to depth D1 and depth D2 to obtain the elemental content of the test sample at etching depths D1 and D2, and further calculate the Mn / Fe molar ratio and M / (Fe + Mn) molar ratio of the test sample at etching depths D1 and D2. Among them, depth D1 is R / 10, depth D2 is 17R / 20, and R is the radius of the positive electrode material particles.

[0037] In some embodiments, in addition to doping with element M, the matrix material is also doped with trace amounts of transition metal elements, and the trace transition metal elements can further increase the structural stability of the positive electrode material. In one example, the trace-doped transition metal element can be element Ti, and the doping content of element Ti in the positive electrode material is 1500 - 6000 ppm. For example, the doping content of element Ti can be 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm or 6000 ppm.

[0038] In some embodiments, the mass content of the coating layer in the positive electrode material is 1.0 - 3.0 wt%, for example, the mass content of the coating layer can be 1.0 wt%, 1.5 wt%, 1.7 wt%, 2.0 wt%, 2.5 wt% or 3.0 wt%. Optionally, the mass content of the coating layer can be 1.5 - 1.7 wt%.

[0039] In some embodiments, the particle size of the positive electrode material is 100 - 600 nm. For example, the particle size of the positive electrode material can be 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, or 600 nm. In the prior art, to improve the ionic conductivity of lithium iron manganese phosphate materials, the method of reducing the particle size of lithium iron manganese phosphate materials is usually adopted. Although this increases the specific surface area of lithium iron manganese phosphate materials, it will also reduce the tap density of lithium iron manganese phosphate materials and increase the processing difficulty of lithium iron manganese phosphate materials in the battery. In contrast, the positive electrode material with uniform element distribution in this application has uniform particles and can ensure the tap density of the material at a small particle size. Therefore, when the positive electrode material in this application is within the above particle size range, it can have both a high specific surface area and tap density, and can effectively improve the rate performance and cycle performance of the secondary battery.

[0040] The inventors' research found that currently, for the large-scale production of lithium iron manganese phosphate positive electrode materials, the prior art usually uses the high-temperature solid-phase method to sinter and prepare lithium iron manganese phosphate materials. However, the uniformity of the distribution of Mn and Fe atoms in the lithium iron manganese phosphate materials prepared by this preparation method is poor, which leads to a decrease in the voltage platform and deterioration of the cycle performance during the use of the battery. Therefore, to solve the above problems, the present invention provides a preparation method for the positive electrode material described in any one of the above embodiments. This preparation method first prepares a homogeneous precursor of lithium iron manganese phosphate by the coprecipitation method, and then sinters the homogeneous precursor of lithium iron manganese phosphate to prepare a lithium iron manganese phosphate material with good uniformity of element distribution. This preparation method, while retaining the advantages of large-scale production of the sintering method, is beneficial to improving the problem of uneven distribution of Mn and Fe atoms in lithium iron manganese phosphate, and while improving the electrochemical performance of the material, it also helps to improve the problem of low tap density of lithium iron manganese phosphate materials and improve the processing performance of the positive electrode material in the secondary battery.

[0041] Specifically, please refer to Figure 1 , the preparation method of the above positive electrode material includes the following steps:

[0042] S1. Using the coprecipitation method, prepare a manganese iron phosphate precursor doped with element M or not doped with element M according to the set component ratio;

[0043] S2. Mix and sinter the manganese iron phosphate precursor with a lithium source and a carbon source to obtain a positive electrode material;

[0044] Among them, element M includes at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca.

[0045] The preparation method first prepares a homogeneous precursor of iron manganese phosphate with relatively uniform element distribution in the finished product particles by coprecipitation method. The molar contents of elements such as Mn, Fe, and P in the homogeneous precursor of iron manganese phosphate tend to be consistent at different positions. Then, the homogeneous precursor of iron manganese phosphate is mixed and sintered with a lithium source and a carbon source to obtain a cathode material, which includes a lithium iron manganese phosphate matrix and a carbon coating layer coated on the surface of the matrix. Since the sintering reaction between the homogeneous precursor of iron manganese phosphate and the lithium source does not cause too much change in the element distribution in the homogeneous precursor of iron manganese phosphate, the Fe and Mn elements in the synthesized lithium iron manganese phosphate matrix also have good distribution uniformity in the material particles.

[0046] In some embodiments, step S1 uses the coprecipitation method to prepare an iron manganese phosphate precursor without doping element M according to a set component ratio. This step S1 specifically includes the following steps:

[0047] Mix a manganese source, an iron source, and a phosphorus source in a molar ratio of Mn:Fe:P of (10 - 20):(0.01 - 10):20, and carry out a coprecipitation reaction to obtain an iron manganese phosphate precursor without doping element M. This iron manganese phosphate precursor is NH4Mn x Fe y PO4 or Mn x Fe y PO4·H2O, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0.8 ≤ x + y ≤ 1.2. In the particles of this iron manganese phosphate precursor, the distribution uniformity of Fe and Mn atoms is good, and the synthesized cathode material using this iron manganese phosphate precursor also has good element distribution uniformity.

[0048] In other embodiments, step S1 uses the coprecipitation method to prepare an iron manganese phosphate precursor doped with element M according to a set component ratio. This step S1 specifically includes the following steps:

[0049] Mix a manganese source, an iron source, an M source, and a phosphorus source in a molar ratio of Mn:Fe:M:P of (10 - 20):(0.01 - 10):(0.01 - 4):20, and carry out a coprecipitation reaction to obtain an iron manganese phosphate precursor doped with element M. This iron manganese phosphate precursor is NH4Mn x Fe y M z PO4 or Mn x Fe y M zPO4·H2O, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 < z ≤ 0.2, and 0.8 ≤ x + y + z ≤ 1.2. In the particles of the manganese iron phosphate precursor, the distribution uniformity of Fe, Mn, and M atoms is relatively good, and the positive electrode material sintered using the manganese iron phosphate precursor also has relatively good element distribution uniformity.

[0050] In some embodiments, the manganese source may include any type of water-soluble material and may include manganese-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides, or hydroxyoxides. Specifically, the manganese source may be selected from at least one of Mn(NO3)2, MnC2O4, MnCl2, and Mn(SO4)2. Optionally, the manganese source may be MnC2O4.

[0051] In some embodiments, the iron source may include any type of water-soluble material and may include iron-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides, or hydroxyoxides. Specifically, the iron source may be selected from at least one of Fe(NO3)2, FeC2O4, FeCl2, and Fe(SO4)2. Optionally, the manganese source may be FeC2O4.

[0052] In some embodiments, the phosphorus source may be selected from at least one of NH4H2PO4 and (NH4)2HPO4.

[0053] In some embodiments, the M source may be any water-soluble material containing the M element and may include, for example, M element-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides, or hydroxyoxides. Specifically, the M salt may include at least one of MCO3, M(NO3)2, M(NO2)2, M(OH)2, M(OH)2·H2O, MC2O4, MH2, MF2, MCl2, MBr2, MI2, (CH3COO)2M, and MSO4; preferably, the M source may be MC2O4.

[0054] In some embodiments, step S2 uses a secondary sintering method to prepare the positive electrode material, which can further achieve uniform compounding of various elements in the lithium manganese iron phosphate matrix and can also achieve uniform coating of the carbon material on the surface of the lithium manganese iron phosphate matrix particles, thereby improving the electrochemical performance of the positive electrode material.

[0055] Specifically, as Figure 2 shown, step S2 includes the following steps:

[0056] S21. Mix the manganese iron phosphate precursor, the lithium source, and the carbon source and perform a primary sintering to obtain a composite material of the matrix and carbon; the matrix is lithium manganese iron phosphate doped with the M element or not doped with the M element.

[0057] Specifically, first dissolve the carbon source in deionized water; then mix the lithium source and the lithium iron phosphate manganese precursor in a molar ratio of Li:P of 0.9 to 1.3, and successively carry out wet grinding and drying on the mixture of the lithium source and the lithium iron phosphate manganese precursor. Add the dried mixture to deionized water to obtain a mixed solution; place the mixed solution in an inert atmosphere and carry out primary sintering at a temperature of 500 - 780 °C to obtain a composite material of matrix and carbon.

[0058] S22. Carry out secondary sintering on the composite material to form a coating layer on the surface of the matrix, so that the coating layer covers at least part of the surface of the matrix to obtain a positive electrode material; the coating layer is a carbon coating layer;

[0059] Specifically, crush, wet grind and dry the composite material, and place it in an inert atmosphere after drying and carry out secondary sintering at a temperature of 500 - 780 °C. After sintering, crush the material to obtain a positive electrode material.

[0060] In some embodiments, the lithium source can include any type of material soluble in water, and can include, for example, lithium-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides or hydroxyoxides. Specifically, the lithium salt can include at least one of Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, Li2C2O4, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi·2H2O or Li3C6H5O7; optionally, the lithium salt can be selected as Li2CO3.

[0061] In some embodiments, the carbon source is an organic carbon source, and the carbon source is selected from at least one of glucose, fructose, polyethylene glycol, polyvinyl alcohol, sucrose, galactose, polyvinylpyrrolidone (PVP), tannic acid, cellulose, citric acid and ascorbic acid.

[0062] The present invention also provides an electrochemical device, which can be a solid-state lithium-ion secondary battery or a liquid lithium-ion secondary battery. Taking the liquid lithium-ion secondary battery as an example, the electrochemical device includes a positive electrode plate, a negative electrode plate, a separator disposed between the positive electrode plate and the negative electrode plate, and an electrolyte. The positive electrode plate includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer includes a conductive agent, a binder, and the positive electrode material described in any one of the above embodiments, or the positive electrode material prepared by the preparation method described in any one of the above embodiments; the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer includes a negative electrode material, a conductive agent, a thickening agent, and an adhesive. Among them, the positive and negative electrode materials can intercalate and deintercalate lithium ions to achieve energy storage and release. The electrolyte is a carrier for lithium ions to transport between the positive and negative electrodes. The separator can permeate lithium ions but is non-conductive, thereby separating the positive and negative electrodes to prevent short circuit.

[0063] It should be noted that the preparation of the positive electrode plate, negative electrode plate, separator, and electrolyte in the electrochemical device and the assembly process of the electrochemical device can be carried out by conventional methods in the art. The preparation method of the electrochemical device is described by way of example below:

[0064] (1) Preparation of the positive electrode plate: The positive electrode material, conductive agent, and binder are mixed in a weight ratio of (90 to 99):(1 to 10):(1 to 10). Optionally, the weight ratio can be 93:3:4. Solvent N-methylpyrrolidone (NMP) is added and stirred thoroughly to obtain a positive electrode slurry; under the action of a vacuum mixer, it is stirred until the system becomes homogeneous and transparent to obtain the positive electrode slurry. The positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and then the positive electrode current collector aluminum foil is air-dried at room temperature and transferred to an oven for drying, and then obtained the positive electrode plate through cold pressing and slitting. Among them, the conductive agent can be selected from at least one of conductive materials such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, and vapor grown carbon fiber (VGCF). For example, the conductive agent is SP and CNT, and the mass ratio of SP and CNT is 2:1; the binder can be selected from at least one of polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), etc. For example, the binder can be PVDF.

[0065] (2) Preparation of the negative electrode sheet: After mixing the negative electrode material, conductive agent, thickening agent, and binder in a mass ratio of 97:1:1:1, deionized water is added, and the solid content of the slurry is adjusted to 55%. Then, under the action of a vacuum mixer, the mixture is sufficiently stirred to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on both sides of an 8-μm negative electrode current collector copper foil; after drying at room temperature, it is transferred to an oven for drying, and then through processes such as cold pressing and slitting, a negative electrode sheet is obtained. Among them, the negative electrode material is selected from one or more of artificial graphite, natural graphite, soft carbon, hard carbon, pure silicon, silicon oxide compounds, and silicon carbide compounds. The conductive agent can be selected from at least one of conductive materials such as carbon black (SuperP), acetylene black, carbon nanotubes (CNT), graphene, and vapor grown carbon fiber (VGCF). The binder is selected from at least one of binder materials such as polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), and styrene-butadiene rubber (SBR); the thickening agent is selected from carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0066] (3) Preparation of the electrolyte: In a glove box under an argon atmosphere with a water content of <10 ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are uniformly mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, the fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0067] (4) Preparation of the separator: The separator is selected from the conventional types in the art. For example, a PE porous membrane is selected as the separator. The thickness of the separator is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL; the porosity is 30% to 50%.

[0068] (5) Assembly of the battery: The battery is assembled according to the conventional method. For example: After preparation, the negative electrode sheet, separator, and positive electrode sheet are stacked in sequence, and then placed into an aluminum-plastic film to obtain a dry battery cell, and the dry battery cell is baked to remove water. The prepared electrolyte is injected into the dry battery cell and sealed to obtain a finished lithium-ion battery.

[0069] The technical solutions of the present invention will be described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available products or can be prepared by conventional methods in the art.

[0070] Example 1

[0071] This example provides a positive electrode material, which includes a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.35 Ni 0.05PO4, the coating layer includes amorphous carbon, and the mass content of the coating layer in the cathode material is 1.5 wt%. In addition, a trace amount of Ti element is doped in the cathode material, and the doping content of Ti element in the cathode material is 5000 ppm. The preparation method of the cathode material is as follows:

[0072] S1. Mix manganese oxalate, iron oxalate, nickel oxalate and ammonium dihydrogen phosphate according to the molar ratio of Mn:Fe:Ni:P of 12:7:1:20, and carry out a coprecipitation reaction to obtain the manganese iron phosphate precursor NH4Mn 0.6 Fe 0.35 Ni 0.05 PO4.

[0073] S2. Dissolve glucose in deionized water; mix lithium carbonate, the manganese iron phosphate precursor and titanium dioxide, where the Li:P molar ratio of lithium carbonate to the manganese iron phosphate precursor is 1.2:1, and carry out wet grinding and drying. Then add the dried mixture of lithium carbonate and the manganese iron phosphate precursor to deionized water to obtain a mixed solution; place the mixed solution in an inert atmosphere furnace and sinter it at a constant temperature of 700 °C for 10 hours to obtain a composite material of matrix LiMn 0.6 Fe 0.35 Ni 0.05 PO4 and carbon; after crushing, wet grinding and drying the composite material, place it in an inert atmosphere furnace and sinter it at a constant temperature of 760 °C for 12 hours, and crush it after sintering to obtain the cathode material.

[0074] Example 2

[0075] The cathode material prepared in this example has the same system as that in Example 1. The cathode material includes a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.35 Ni 0.05 PO4, the coating layer includes amorphous carbon, and the mass content of the coating layer in the cathode material is 1.5 wt%. The difference between this example and Example 1 is that titanium dioxide is not added to the primary sintering raw materials in step S2 of this example, and there is no Ti element doping in the prepared cathode material.

[0076] Example 3

[0077] This example provides a cathode material, which includes a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.4 PO4, the coating layer includes amorphous carbon, and the mass content of the coating layer in the cathode material is 1.5 wt%. In addition, a trace amount of Ti element is doped in the cathode material, and the doping content of Ti element in the cathode material is 5000 ppm. The preparation method of the cathode material is as follows:

[0078] S1. Mix manganese oxalate, iron oxalate, and ammonium dihydrogen phosphate in a molar ratio of Mn:Fe:P = 3:2:5, and carry out a coprecipitation reaction to obtain the manganese iron phosphate precursor NH4Mn 0.6 Fe 0.4 PO4.

[0079] S2. Dissolve glucose in deionized water; mix lithium carbonate, the manganese iron phosphate precursor, and titanium dioxide, where the Li:P molar ratio of lithium carbonate to the manganese iron phosphate precursor is 1.2:1, and carry out wet grinding and drying. Then, add the dried mixture of lithium carbonate and the manganese iron phosphate precursor to deionized water to obtain a mixed solution; place the mixed solution in an inert atmosphere furnace and sinter it at a constant temperature of 700 °C for 10 hours to obtain a composite material of matrix LiMn 0.6 Fe 0.4 PO4 and carbon; crush, wet grind, and dry the composite material, then place it in an inert atmosphere furnace and sinter it at a constant temperature of 760 °C for 12 hours. After sintering, crush it to obtain the cathode material.

[0080] Example 4

[0081] This example prepares a cathode material with the same system as in Example 3. The cathode material includes a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.4 PO4, and the coating layer includes amorphous carbon. The mass content of the coating layer in the cathode material is 1.5 wt%. In addition, a trace amount of Ti element is doped in the cathode material, and the doping content of the Ti element in the cathode material is 5000 ppm. The difference between this example and Example 3 is that in step S2, the constant temperature sintering time for the first sintering is 22 hours, and there is no second sintering.

[0082] Example 5

[0083] This example provides a cathode material, which includes a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.35 Mg 0.05 PO4, and the coating layer includes amorphous carbon. The mass content of the coating layer in the cathode material is 1.5 wt%. In addition, a trace amount of Ti element is doped in the cathode material, and the doping content of the Ti element in the cathode material is 5000 ppm. The preparation method of this cathode material is as follows:

[0084] S1. Mix manganese oxalate, iron oxalate, magnesium oxalate, and ammonium dihydrogen phosphate in a molar ratio of Mn:Fe:Mg:P = 12:7:1:20, and carry out a coprecipitation reaction to obtain the manganese iron phosphate precursor NH4Mn 0.6 Fe 0.35 Mg 0.05 PO4.

[0085] S2. Dissolve glucose in deionized water; mix lithium carbonate, lithium iron manganese phosphate precursor and titanium dioxide, where the Li:P molar ratio of lithium carbonate and lithium iron manganese phosphate precursor is 1.2:1, and perform wet grinding and drying. Then add the dried mixture of lithium carbonate and lithium iron manganese phosphate precursor to deionized water to obtain a mixed solution; place the mixed solution in an inert atmosphere furnace and sinter it at a constant temperature of 700 °C for 10 hours to obtain the matrix LiMn 0.6 Fe 0.35 Mg 0.05 PO₄ composite with carbon; after crushing, wet grinding and drying the composite, place it in an inert atmosphere furnace and sinter it at a constant temperature of 760 °C for 12 hours, and crush it after sintering to obtain the cathode material.

[0086] Comparative Example 1

[0087] Comparative Example 1 provides a cathode material, which has a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.35 Ni 0.05 PO₄, and the coating layer includes amorphous carbon. The mass content of the coating layer in the cathode material is 1.5 wt%. In addition, a trace amount of Ti element is doped in the cathode material, and the doping content of Ti element in the cathode material is 5000 ppm. The preparation method of this cathode material is as follows:

[0088] S1. First, dissolve glucose and ammonium dihydrogen phosphate in deionized water, then mix ferrous oxalate, manganese oxalate, nickel oxalate, lithium carbonate and titanium dioxide, grind them and add them to deionized water to obtain a mixed solution; among them, the Mn:Fe:Ni:P:Li molar ratio of manganese oxalate, ferrous oxalate, nickel oxalate, ammonium dihydrogen phosphate and lithium carbonate is 12:7:1:20:24.

[0089] S2. Place the mixed solution in an inert atmosphere furnace and sinter it at a constant temperature of 700 °C for 10 hours to obtain the matrix LiMn 0.6 Fe 0.35 Ni 0.05 PO₄ composite with carbon; after crushing, wet grinding and drying the composite, place it in an inert atmosphere furnace and sinter it at a constant temperature of 760 °C for 12 hours, and crush it after sintering to obtain the cathode material.

[0090] Comparative Example 2

[0091] Comparative Example 2 provides a cathode material, which has a matrix and a coating layer. The matrix is LiMn 0.6 Fe 0.4PO4, the coating layer includes amorphous carbon, and the mass content of the coating layer in the cathode material is 1.5 wt%. In addition, a trace amount of Ti element is doped in the cathode material, and the doping content of Ti element in the cathode material is 5000 ppm. The preparation method of the cathode material is as follows:

[0092] S1. First, dissolve glucose and ammonium dihydrogen phosphate in deionized water, and then mix and grind ferrous oxalate, manganese oxalate, lithium carbonate and titanium dioxide and add them to deionized water to obtain a mixed solution; wherein, the molar ratio of Mn:Fe:P:Li of manganese oxalate, ferrous oxalate, nickel oxalate, ammonium dihydrogen phosphate and lithium carbonate is 12:8:20:24.

[0093] S2. Place the mixed solution in an inert atmosphere furnace and sinter it at a constant temperature of 700 °C for 10 hours to obtain a composite material of matrix LiMn 0.6 Fe 0.4 PO4 and carbon; after crushing, wet grinding and drying the composite material, place it in an inert atmosphere furnace and sinter it at a constant temperature of 760 °C for 12 hours, and crush it after sintering to obtain the cathode material.

[0094] Assemble the cathode materials prepared in Examples 1 to 5 and Comparative Examples 1 to 2 into coin-type half-cells respectively, and conduct compaction density tests, discharge capacity retention tests and cycle capacity retention tests on the cathode materials prepared in Examples 1 to 5 and Comparative Examples 1 to 2 through the coin-type half-cells. The test results are shown in Table 1 to verify the efficacy of the present invention.

[0095] Conduct a compaction density test on the cathode electrode sheet, and the test method is as follows:

[0096] Take the cold-pressed cathode electrode sheet, and measure that the area of the cold-pressed electrode sheet is s (mm 2 ), the thickness of the electrode sheet is h1 (mm), the weight of the electrode sheet is m (g), and the thickness of the positive current collector aluminum foil is h2 (mm). Then the compaction density PD of the cathode electrode sheet = 1000m / s(h1 - h2) (unit: g / cm 3 )

[0097] The preparation process of the coin-type half-cell is as follows: The above-prepared cathode material, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) are mixed in a weight ratio of 8:1:1, and the solvent N-methylpyrrolidone (NMP) is added, and they are fully stirred and mixed to obtain the cathode slurry; the cathode slurry is coated on carbon-coated aluminum foil, and after drying, a cathode electrode sheet with a diameter of Φ12mm is made; a lithium metal is used as the anode electrode sheet, and a PE porous membrane is used as the separator, and the cathode shell, cathode electrode sheet, separator, anode electrode sheet, and anode shell are assembled into a half-cell in a glove box in sequence. Among them, 35 μL of electrolyte is dropped on both sides of the separator, and it is placed at room temperature for 24 h to allow the electrolyte to fully infiltrate the electrode sheet, and then the subsequent battery tests are carried out.

[0098] The coin-type half-cell is subjected to charge-discharge tests to test the rate performance (0.1C / 1C charge-discharge capacity retention rate) and cycle performance (200cls capacity retention rate) of the cathode material. The test method is as follows:

[0099] Rate performance test: In the test voltage range of 2.8V (discharge cut-off voltage) to 4.3V (charge cut-off voltage), the coin-type half-cell is charged at a constant current and constant voltage at a current rate of 0.1C (the charge cut-off current rate is 0.05C), and the charge capacity is measured after full charge; then it is discharged at a constant current and constant voltage at a current rate of 1C, and the discharge capacity is measured after discharge. The 0.1C / 1C charge-discharge capacity retention rate can be obtained by dividing the 1C discharge capacity by the 0.1C charge capacity.

[0100] Cycle performance test: At an ambient temperature of 45°C, in the test voltage range of 2.8V (discharge cut-off voltage) to 4.3V (charge cut-off voltage), the coin-type half-cell is charged and discharged at a constant current and constant voltage at a current rate of 1C for 200 cycles (the charge cut-off current rate is 0.05C), and the ratio of the discharge capacity per gram of the 200th cycle of the coin-type half-cell to the discharge capacity per gram of the first cycle is recorded as the 200cls cycle capacity retention rate.

[0101] The performance test results of the cathode materials in Examples 1 to 5 and Comparative Examples 1 to 2 and the corresponding assembled coin-type half-cells are shown in Table 1.

[0102] Table 1: Performance test results of the cathode materials in Examples 1 to 5 and Comparative Examples 1 to 2.

[0103]

[0104] Comparing the test results of Example 1 and Comparative Example 1, as well as Example 3 and Comparative Example 2, it can be seen that compared with the lithium iron manganese phosphate cathode material prepared by high-temperature solid-phase sintering, the lithium iron manganese phosphate cathode material prepared by using the precursor prepared by the coprecipitation method has a more uniform element distribution, good structural stability, smoother ion migration in the material, the assembled cathode electrode has a higher tap density, and the 1C discharge capacity retention rate and 200cls cycle capacity retention rate of the assembled battery are further improved.

[0105] Comparing the test results of Example 1 and Example 2, it can be seen that doping a trace amount of Ti element in the lithium iron manganese phosphate cathode material helps to improve the ionic conductivity and structural stability of the cathode material, thereby further enhancing the discharge capacity, rate performance and cycle performance of the assembled battery.

[0106] Comparing the test results of Example 3 and Example 4, it can be seen that at the same sintering time, compared with single coating sintering, using double coating sintering in the preparation process of the cathode material can more effectively achieve uniform coating of lithium iron manganese phosphate particles, which is beneficial to improving the tap density and conductivity of the cathode material, thereby further enhancing the rate performance and cycle performance of the assembled battery.

[0107] In summary, the present invention provides a cathode material. In the lithium iron manganese phosphate matrix of this cathode material, Mn and Fe atoms are evenly distributed. On the one hand, it helps to increase the tap density of the lithium iron manganese phosphate material, improve the processing performance of the cathode material, and increase the discharge capacity of the battery; on the other hand, it can also stabilize the crystal structure of the lithium iron manganese phosphate material, making it difficult for Mn ions to distort, and avoiding hindrance to the insertion and extraction of Li ions in the material, thereby effectively enhancing the discharge capacity, rate performance and cycle performance of the cathode material.

[0108] The above embodiments are only illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Any person familiar with this technology can modify or change the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes completed by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A cathode material, characterized in that, Comprising: Substrate, the substrate includes LiMn x Fe y M z PO4, 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.2, 0.8 ≤ x + y + z ≤ 1.2, the M element includes at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca; A coating layer, the coating layer comprising amorphous carbon; Among them, in the particles of the positive electrode material, the ratio of the Mn / Fe molar ratio at a depth D1 from the particle surface to the Mn / Fe molar ratio at a depth D2 from the particle surface is 0.9 to 1.1; the depth D1 is R / 10, the depth D2 is 17R / 20, and the R is the radius of the particle.

2. The cathode material according to claim 1, characterized in that, The matrix satisfies 0 < z ≤ 0.2; and in the particles of the positive electrode material, the ratio of the molar ratio of M / (Fe + Mn) at a depth D1 from the particle surface to the molar ratio of M / (Fe + Mn) at a depth D2 from the particle surface is 0.9 to 1.

1.

3. The cathode material according to claim 1, wherein The mass content of the coating layer in the positive electrode material is 1.0 - 3.0 wt%.

4. The cathode material according to claim 1, wherein The particle size of the positive electrode material is 100 - 600 nm.

5. A method for preparing the cathode material according to any one of claims 1 to 4, characterized in that, Comprising: Using the co-precipitation method, a manganese iron phosphate precursor doped with M element or not doped with M element is prepared according to a set component ratio; Mixing and sintering the manganese iron phosphate precursor with a lithium source and a carbon source to obtain a positive electrode material; Wherein, the M element includes at least one of Ti, V, Sc, Cr, Co, Ni, Cu, Zn, Mo, Ag, W, Mg, Al, and Ca.

6. The preparation method according to claim 5, wherein The using the co-precipitation method to prepare a manganese iron phosphate precursor not doped with M element according to a set component ratio includes: Mixing a manganese source, an iron source, and a phosphorus source in a molar ratio of Mn:Fe:P of (10 - 20):(0.01 - 10):20, and performing a co-precipitation reaction to obtain a manganese iron phosphate precursor not doped with M element.

7. The preparation method according to claim 5, characterized in that, The using the co-precipitation method to prepare a manganese iron phosphate precursor doped with M element according to a set component ratio includes: Mixing a manganese source, an iron source, an M source, and a phosphorus source in a molar ratio of Mn:Fe:M:P of (10 - 20):(0.01 - 10):(0.01 - 4):20, and performing a co-precipitation reaction to obtain a manganese iron phosphate precursor doped with M element.

8. The preparation method according to any one of claims 5 to 7, characterized in that, The manganese iron phosphate precursor is NH4Mn x Fe y M z PO4 or Mn x Fe y M z PO4·H2O, where 0.5 < x ≤ 1.0, 0 < y ≤ 0.5, 0 ≤ z ≤ 0.2, and 0.8 ≤ x + y + z ≤ 1.

2.

9. The preparation method according to claim 5, characterized in that, The mixing and sintering the manganese iron phosphate precursor with a lithium source and a carbon source to obtain a positive electrode material includes: Mixing the manganese iron phosphate precursor, a lithium source, and a carbon source and performing a primary sintering to obtain a composite material of a matrix and carbon; the matrix is manganese iron lithium phosphate doped with M element or not doped with M element; Performing a secondary sintering on the composite material to form a coating layer on the surface of the matrix, so that the coating layer coats at least part of the surface of the matrix to obtain a positive electrode material; the coating layer is a carbon coating layer.

10. An electrochemical device, characterized in that, Comprising a positive electrode plate, the positive electrode plate comprising a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer comprising the positive electrode material according to any one of claims 1 to 4, or the positive electrode material prepared by the preparation method according to any one of claims 5 to 9.

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